The present invention relates the means for the biological treatment of liquid suspensions containing oxidizable matters, and has specific reference to a plant for performing this treatment intended more particularly but not exclusively for the purification of waste water, to the biological treatment of industrial processing water and to the treatment of fermentation liquors.
For many decennaries the conventional source of oxygen for such biological treatment processes was atmospheric air. However, it is now well known, for example in the treatment of waste water by the so-called activated-sludge method, to use as an oxidizing agent a gas carrying oxygen under a partial pressure higher than the atmospheric value, which is also referred to as oxygen-enriched air or pure oxygen, so that, by simply increasing the oxygen partial pressure of the injected gas, relatively high oxygenation capacities, for instance higher than 500 grams per hour of oxygen per cubic meter of tank capacity, can be attained, while utilizing relatively low gas outputs. In many cases to minimize losses of this high-oxygen gas and achieve the best possible diffusion efficiency, the liquid mass to be enriched with oxygen is contained in an enclosed, covered space.
Thus, it is possible under these conditions and in the specific case of the activated-sludge process to maintain a high microorganism concentration in the biological reactor, and to cause the waste water to be purified, the substratum to be oxidized and to contact the highly concentrated activated sludge of which the oxygen demand is considerably greater than that of activated sludge developed in a treatment plant utilizing only atmospheric air. Therefore, the organic matter is oxidized and degraded more rapidly and the necessary contact times are shorter.
However, up to now, two technological difficulties reduced considerably the practical and economical advantage resulting from the use of pure oxygen, notably in waste water biological treatment plants:
a. The biological flock problem. After contacting the oxygenated bacteria, the biological flock must be removed from the then purified water. As a rule, this separation takes place by settling or decantation. Now, decanting a more concentrated liquor requires more time and slower upward flow rates, so that the reduction in the volume of the biological reactor which is afforded by the use of oxygen can be at least partially compensated by increasing the dimensions of the decanter of settling tank. Obviously, the use of an oxygen-enriched gas improves somewhat the physical properties of the sludge (settling propensity) in comparison with the conventional process utilizing atmospheric air, but in most instances this gain is not sufficient.
In either case, this inconvenience is compensated by substituting floating for settling, but this palliative is not applicable under all circumstances and in addition it implies a higher power consumption.
b. The other inconvenience is found in the difficulty of properly regulating or metering the supply of oxygen or oxygen-enriched air to the plant.
In its conventional principle this plant comprises a plurality of oxidation compartments such as open tanks disposed in series.
The liquid to be treated is fed to the upstream tank via a duct, and oxygen is supplied thereto via another duct equipped with automatic valve means actuated by a pressure outlet provided in the gaseous sheet so that the valve opens when the pressure in the tank drops below a predetermined minimum value. These tanks communicate with each other through openings formed in their mutual partitions and comprise means for aerating the liquor. Opening into the downstream tank, above the level attained by the liquor, is a pipe having inserted along its path an automatic valve adapted to close when the oxygen content of the gaseous phase of the downstream compartment drops below the predetermined limit value.
The treated liquid is discharged from the downstream compartment via a pipe into a settling tank. The settled sludge is recycled to the upstream tank via another pipe line which in this example is branched off the duct supplying the liquid to be treated.
In these known plants the oxygen stream flows from the upstream tank to the downstream tank in the same direction as the liquid to be purified and the sludge recycled for a subsequent reactivation.
However, this arrangement is objectionable in that since the oxygen demand and therefore the oxygen consumption decreases from the upstream tank to the downstream tank, when the partial oxygen pressure in the gaseous phase of the downstream tank attains the minimum threshold or alarm value and the valve for venting the tank to the atmosphere opens, the partial oxygen pressures in the gaseous phases of the upstream compartments are also low and must be renewed. Under these conditions it is clear that the supply of oxygen through the upstream compartment valve is extremely irregular.